1. Field of the Invention
This invention relates to a transfer device included in an image forming apparatus, method and image forming apparatus using the same.
2. Discussion of the Background
In an image forming apparatus, such as for example a copying machine, a printer, a facsimile machine or the like, a transfer device, such as for example a transfer roller, is provided to transfer a toner image formed on an image carrier of the apparatus to a transfer sheet conveyed to a transfer position between the image carrier and the transfer device. Relating to the image forming apparatus having a transfer roller, Japanese Patent Publication No. 2686267 of 1997 describes an apparatus with a transmission device which transmits driving force from an image carrier to a transfer roller such that a circumferential speed of the transfer roller is greater than that of the image carrier at a nip part between the transfer roller and the image carrier. With the above described configuration, an image is transferred to a transfer sheet without shifting even when the transfer sheet receives an impact while the image is transferred.
Furthermore, it is known that a transfer error such that a part of a character image is not transferred and thereby the character image has a blank portion therein can be reduced by creating a difference in the circumferential speed between the image carrier and the transfer device such as the transfer roller.
Generally, the transfer roller includes a metal shaft and an elastic body such as conductive foamed polyurethane surrounding the shaft. Because a nip part between the image carrier and a transfer surface of the transfer roller affects transfer performance, the tolerance of an outside diameter of the transfer roller must be accurately controlled. For this reason, generally in the process of making the transfer roller the elastic body is attached to the metal shaft with the outside diameter of the transfer roller made greater than desired, and then the elastic body is ground with a grindstone to have a desired outside diameter.
In the above described grinding process, it is known that, as illustrated in FIG. 8, notches 101 are formed on the transfer surface of a transfer roller 10 such that each notch slants at a certain angle relative to a direction perpendicular to the transfer surface of the transfer roller 10. A slanting direction of the notches 101 is hereinafter called a "notch direction" or a "direction of notches".
It is further known that the conveying force of the transfer roller 10 having the notches 101 falls over time when, as illustrated in FIG. 9, a photoconductive drum 1 as an image carrier and the transfer roller 10 rotate in the directions indicated by arrows C and D, respectively, a transfer sheet 8 as a recording material passes through a nip part between the photoconductive drum 1 and the transfer roller 10 in the direction indicated by an arrow B, the transfer roller 10 rotates with a circumferential speed greater than that of the photoconductive drum 1, and the notch 101 of the transfer roller 10 is slanted toward the moving direction of the transfer sheet 8 indicated by the arrow B. As a result of the reduction of the conveying force of the transfer roller 10, the following transfer errors occur: (1) the length of an image transferred to the transfer sheet 8 in the sheet advancing direction is magnified and reduced from that of an image formed on the image carrier 1 (hereinafter referred to as "magnification/reduction error"), (2) only a peripheral part of an image is transferred to the transfer sheet 8 but some portions in the center of the image are not transferred (hereinafter referred to as "blank image"), and (3) a position of the transferred image is shifted on the transfer sheet 8 (hereinafter referred to as "image shift").
The inventors have found a cause of the above-described transfer errors through intense study, as follows. Generally, a transfer sheet is attracted to a surface of a photoconductive drum as an image carrier by the electrostatic attraction force of the photoconductive drum and is thereby moved by the rotation of the drum. Therefore, when the transfer roller and the photoconductive drum rotate such that the circumferential speed of the transfer roller is greater than that of the photoconductive drum, the transfer sheet relatively moves in the direction opposite to the advancing direction of the transfer roller. When the slanting direction of the notch 101 of the transfer roller 10 is directed in the transfer sheet advancing direction indicated by the arrow B as illustrated in FIG. 9, alien substances around the facing part of the transfer sheet 8 and the transfer roller 10 are prone to enter the concave portion of the notch 101. For example, toner remaining on a part of the photoconductive drum 1 corresponding to a background portion of the transferred image, and paper dust enter the notch 101. Further, when the transfer sheet 8 to be conveyed to the transfer position jams and is not conveyed to the transfer position, a toner image formed on the surface of the photoconductive drum 1 directly contacts the transfer surface of the transfer roller 10, so that the toner enters the concave portion of the notch 101.
Furthermore, in the condition illustrated in FIG. 9, the slanting direction of the notch 101 is opposed to the moving direction of the transfer sheet 8 relative to the transfer roller 10, and the transfer roller 10 rubs against the backside of the transfer sheet 8. Thereby, paper dust is prone to be produced at the transfer position and enters the concave portion of the notch 101.
As a result of the above-described alien substances entering the concave portion of the notch 101, the frictional force of the transfer roller 10 acting on the transfer sheet 8 falls over time, so that the conveying force of the transfer roller 10 falls. Accordingly, the above-described transfer errors, such as, "magnification/reduction error", "blank image", and "image shift" are apt to occur. These transfer errors are prone to occur not only when the circumferential speed of the transfer roller is greater than that of the photoconductive drum but also when the circumferential speed of the transfer roller is smaller than that of the photoconductive drum, i.e., when there is a difference in the circumferential speed between the transfer roller and the photoconductive drum.
Japanese Patent publication No. 2847927 of 1998 describes another image transfer device included in an image forming apparatus in which a transfer roller has notches on the transfer surface thereof. In this image transfer device, because the transfer material conveying force of the transfer roller increases as the notches of the transfer roller are rubbed and thereby convex portions of the notches are reduced as the printing volume increases, the contact area between the transfer material and transfer roller increases. In order to address image quality problems caused by the increase of transfer material conveying speed, the transfer roller is disposed in relation to a surface of a photoconductive drum so that the transfer roller contacts and rotates in the direction where a surface frictional force of the transfer roller in the circumferential direction is large.